The present invention relates generally to the nitrile synthesis wherein a mixture of an aliphatic alphamonoolefin such as propylene or isobutylene, ammonia and oxygen are interacted over a solid ammoxidation catalyst at elevated temperature producing an unsaturated aliphatic nitrile such as acrylonitrile from propylene or methacrylonitrile from isobutylene. More specifically, the invention relates to an improvement in the control of such synthesis allowing most accurate control of the input gas stream and the accurate and fast determination of conversion, selectivity and yield not only of the desired nitrile product but also of unreacted feed materials and/or by-products some of which are accurate telltales of correct or incorrect operation. Most specifically, the invention relates to an effluent gas sampling, gas sample conditioning and gas analysis system.